Although solving the Schroedinger equation for the entire number of degrees of freedom composing a physical system would always lead to exact predictions of its behaviour, this is far from practical or even possible. The actual implementation of Eq. REF  would require the knowledge of the spectrum and eigenvectors of the many-body Hamiltonian, whose size grows exponentially with the number of degrees of freedom. This very unfavourable scaling law of the Hamiltonian's size makes it possible to tackle only very small system sizes, in the order of 20 particles with only two-level states each, and at massive computational costs. Fortunately, very commonly we are only interested in monitoring the behaviour of a small fraction of the total number of degrees of freedom. In a such a case it is possible to simplify the theoretical treatment of relaxation by employing the theory of open-quantum systems[1]}. In this framework one tries to map the many-body problem into a few-body problem (the degrees of freedom we are interested in) where time evolution is driven by an effective equation of motion that incorporates the action of hidden degrees of freedom, commonly referred to as the bath.
